Jupiter’s Great Red Spot – the largest known vortex in the Solar System about the size of three Earths – is shrinking. Measuring approximately 16 500 km across, it’s the smallest size it has ever been, and scientists are baffled as to why.

In the 1800s, the storm was gauged to be about 41 000 km on its long axis. In 1979, Voyager 1 and Voyager 2 estimated it to be 23 300 km across. In 1995, a Hubble photo showed the long axis of the GRS (as referred to by astronomers) at approximately 20 950 km across. And in a 2009 photo, it was measured at 17 900 km across.

Amateur observations in early 2012 revealed that the GRS was decreasing in size more rapidly than it had been – by 930 km per year – changing its shape from an oval to a circle.

Amy Simon of Nasa’s Goddard Space Flight Center says that from the latest Hubble observations, it is apparent very small eddies are feeding into the storm. “We hypothesised these may be responsible for the accelerated change by altering the internal dynamics and energy of the Great Red Spot,” said Simon.

Simon’s team plans to study the motions of the small eddies and the internal dynamics of the storm to determine whether these eddies can feed or sap momentum entering the upwelling vortex, resulting in this yet unexplained shrinkage.